By 2010, the production capacity of ethanol in the United States has exceeded 12 billion gallons per year, much of which is used as alternative fuel. Most of the production in the United States occurs in dry-mill ethanol plants through fermenting corn as the feedstock. The economic viability of an ethanol plant resides in being able to produce ethanol at low cost to be competitive with other ethanol producers and provide ethanol at a price competitive with gasoline. Accordingly, these ethanol plants are highly energy integrated and have capacities greater than 100 million gallons per year to take advantage of economies of scale.
In conventional ethanol plants the sugar source such as one or more of corn, wheat, sugar beets, oats, barley, sugar cane, sorghum, cassava, rice, and the like is milled and subjected to pretreatment which typically includes an enzymatic hydrolysis to convert starches to sugars. The sugars are then fermented to ethanol using a suitable microorganism such as yeast. The fermentation is generally done in batches with each batch being maintained under fermentation conditions for 2 to 4 days. The duration of the fermentation can be varied. As the batch fermentation progresses, the concentration of sugars declines. The rate of conversion of sugars to ethanol decreases as the sugars become depleted or the ethanol concentration attains a concentration that adversely affects the microorganisms. Thus, the operation of the batch fermentations is typically such that for a given amount of sugars provided to the batch reactor sufficient water is present to assure that when the sought amount of sugars are converted, the concentration of ethanol is below that which is unduly deleterious to the microorganisms.
The fermentation broth which contains in an aqueous medium ethanol and other metabolites, any unconverted sugars, and indigestible solids from the sugar source and the microorganisms and their debris, is passed to a beer still to separate ethanol and provide as a bottom stream a whole stillage containing water, solids and higher boiling components. This whole stillage is separated into a liquid fraction (thin stillage) and a concentrated solids fraction typically called distillers grains. The distillers grains can be used as animal feed, and water is usually recovered from the thin stillage for reuse in the process via an evaporation operation. The concentrated stream from the evaporation operation contains sugars and may be discarded or admixed with the distiller grains. The sugars comprise pentose, i.e., five carbon sugars.
These ethanol plants consume considerable amounts of energy, e.g., for distillation to recover ethanol and to recover water used in the fermentation process for recycling to the process. Numerous proposals exist to improve the economics of ethanol plants. However, the viability of any proposal will need to take into account the effect that any change may have on the highly integrated design and on co-products. Most plants derive significant revenue from co-products which are primarily carbon dioxide, distillers grains which are used as animal feed and corn oil.
The unit operations for an ethanol plant are designed for a given production rate of ethanol. The batch fermenters, as discussed above, have a limited window of operation due to constraints on conversion rates and final ethanol concentration. Thus, any increase in ethanol production rate would result in a greater flow rate of feed to the beer still. The beer still and the available steam are designed for a maximum flow rate. Thus in order to operate within the constraints of an existing ethanol plant, the operator must determine whether the batch fermentation conditions should favor maximizing the rate of production of ethanol per unit time or maximizing the conversion of sugars in the sugar source to ethanol. Where demand for ethanol is high, 3 percent or more, and in some instances up to 7 percent, of the available sugars may be unconverted.
Proposals have been made to “setback” a portion of the thin stillage to avoid the energy costs associated with evaporating water from the thin stillage. However, the ability to setback is limited as components contained in the thin stillage can adversely affect the fermentation. Reaney, et al., in U.S. Published Patent Application 2011/0130586 state that metabolites generated in the saccharification and fermentation tanks end up in the thin stillage and at higher concentration levels can inhibit enzyme activities and microbial metabolism. They disclose removing these metabolites and plant extractives and plant derivatives from the thin stillage.
Dale, in U.S. Patent Application Publication 2009/0227004, proposes metabolizing gums in the whole stillage to facilitate removal of corn oil. The adoption of any such process needs to take into account the equipment to effect required and byproducts of any such fermentation in the process.
Abbas, et al., in U.S. Patent Application Publication 2009/0155414, proposes to enhance ethanol fermentation yields by stopping the fermentation when the broth still contains significant amounts of sugars and recovering sugars from the thin stillage. They state at paragraph 0021:                “Optionally, the nutritional value of the distillers' molasses may be increased by treatment and addition of fiber solubles obtained from the ethanol production process. For example, the corn fiber stream derived from a corn wet mill, or the hull fraction from a corn dry grind process may be subjected to a thermochemical and/or enzymatic treatment to solubilize the fiber. The solubilized fiber will include pentose sugars, including but not limited to D-xylose and L-arabinose and their oligosaccharides.”        “The solubilized fiber fraction can be mixed with the high-sugar backset prior to partial evaporation to form the distillers' molasses. This can enhance the quality of the distillers' molasses, because the sugars and oligomers have been known and used for their probiotic properties that enhance the resulting feed.”        
Balan, et al., in U.S. Published Patent Application 2009/0093027 disclose an integrated process involving the addition of corn stillage to lignocellulosic material such as corn stover, and then converting the lignocellulosic material to ethanol.
Accordingly, a need exists to increase the production rate of an existing ethanol plant without undue capital and operating costs while still obtaining a high conversion rate of sugars to ethanol. Also, a desire exists to provide a fermentation broth to the beer still that contains a higher concentration of ethanol and thereby reduce the amount of steam required for the distillation per unit of ethanol produced.
Another desire of existing ethanol plants is to produce higher value bio-products such as hydrocarbons and other organic products such as propanol, propanediol, butanol, butanediol, lactic acid, and the like without undue capital costs.